Position determination of a movable target in three dimensional space is may be based upon range and/or angle determinations from which the coordinates of the target are determined. In radar based methods radio frequency pulsed radiation is used. The range of the target is determined by noting the round trip time of a pulse from the moment it leaves the radar system until it returns after having reflected of the target. The range is given by the product of half the round trip time times the velocity of light. The shorter the wavelength used the greater the precision of the estimated range of the target from the radar system.
Among the optical methods range estimation using light in free space (i.e., excluding waveguide technologies), the Time Of Flight (TOF) and the interferometry or heterodyne detection methods (see, e.g., Ifftimia N., Kopica M. and Spanulescu I. "Active Remote Sensing Heterodyne Technique," SPIE Vol. 2120, pp. 136-142, 1994) are most commonly used The former is the optical equivalent of the radar method and estimates the distance between the light pulse transmitter/receiver and the target by measuring the round trip time required for a light pulse to travel between the transmitter/receiver and the target. Alternatively if the transmitter and receiver are at different locations the time of flight of the light pulse from the transmitter to the receiver via the target is measured.
The heterodyne detection method uses a coherent light in an interferometer configuration and is generally more accurate than the more straightforward TOF method which is useful for measuring distances from several meters to a few kilometers. For the measurement of distances from few a millimeters to several meters interferometers using a coherent light source can be used. The interferometric method has a drawback in that there is an inherent ambiguity in counting integer fringes. The heterodyne detection method is based on the absolute distance interferometry method in which a synthetic long wavelength is produced by using two wavelengths with a small wavelength difference between them (see, e.g., Yang Z., Ting Z. and Weiming R. "Heterodyne Interferometer with a Dual Mode He-Ne Laser for Absolute Distance Interferometry," SPIE Vol. 2544, pp. 354-357, 1995).
It is an object of the present invention to provide a method for accurate real three dimensional time position determination of a movable target that does not suffer from estimation ambiguities.